This is what our Milky Way galaxy looks like when viewed with neutrinos

Scientists with the IceCube Neutrino Observatory have unveiled a striking new image of our Milky Way galaxy as seen by ghost-like messenger particles called neutrinos. This new analysis—announced at a Drexel University event today, with a paper being published in the journal Science tomorrow—offers the strongest evidence to date that the Milky Way is a source of high-energy neutrinos, shedding more light on the origin of high-energy cosmic rays.

"I remember saying, 'At this point in human history, we're the first ones to see our galaxy in anything other than light,'" said Drexel University physicist and IceCube member Naoko Kurahashi Neilson of the moment she and two graduate students first examined the image. “Observing our own galaxy for the first time using particles instead of light is a huge step. As neutrino astronomy evolves, we will get a new lens with which to observe the universe.”

As previously reported, ever since French physicist Pierre Auger proposed in 1939 that cosmic rays must carry incredible amounts of energy, scientists have puzzled over what produces these powerful clusters of protons and neutrons raining down into Earth's atmosphere. One way to identify the sources is to backtrack the paths that high-energy cosmic neutrinos traveled on their way to Earth since they are created by cosmic rays colliding with matter or radiation, producing particles that then decay into neutrinos and gamma rays.

Most neutrino hunters bury their experiments deep underground, the better to cancel out noisy interference from other sources. In the case of IceCube, the collaboration features arrays of basketball-size optical sensors buried deep within the Antarctic ice. On those rare occasions when a passing neutrino interacts with the nucleus of an atom in the ice, the collision produces charged particles that emit UV and blue photons. Those are picked up by the sensors. So IceCube is well-positioned to help scientists advance their knowledge of the origin of high-energy cosmic rays.

One strong possible source for high-energy cosmic rays is active galactic nuclei (AGNs), found at the center of some galaxies. Their energy arises from supermassive black holes at the galaxy's center and/or from the black hole's spin. It's not an easy task to locate high-energy neutrino sources in space, given the large number of background neutrinos and other particles in the Earth's atmosphere. IceCube records roughly 100 million muons for every single neutrino it detects, for instance. In 2018, IceCube picked up a flare of neutrinos that seemed to be coming from a type of AGN called a blazar. But they needed to find other similar cosmic neutrino sources to reconcile that observation with existing neutrino models.

In 2020, the IceCube collaboration analyzed data collected between 2008 and 2018. They found a tantalizing hint of 63 excess neutrinos coming from the direction of four AGN, although only one—Messier 77 (aka NGC 1068, or the Squid Galaxy)—reached any statistical significance. Even so, it was just 2.9 sigma, short of what's required to claim discovery; it could have simply been a random background fluctuation.

So the IceCube scientists revisited the data again last year, this time incorporating machine-learning techniques to better reconstruct the trajectories and energies of the photons picked up by the detectors. Then they reprocessed that same 10 years of data. The result: an excess of 79 neutrinos over the background, with a statistical significance of 4.2 sigma. So Messier 77 is indeed a strong candidate for one such high-energy neutrino emitter.

But what about our Milky Way galaxy? The denser portions of the Milky Way's inner galactic plane are the most likely places to detect an intense neutrino flux because that density should result in more of the sort of collisions that produce neutrinos. The issue is that IceCube is mainly blind to that portion of the southern sky because neutrinos don't have to traverse the Earth, which helps weed out much of the noise from other particles.

There are two basic patterns of light that IceCube can detect in its channels. One is the track patterns produced when a neutrino interacts with particles in the ice, producing a highly directional flash of light. Those events typically point to a specific area of the sky so it's easier to pinpoint where a neutrino came from. More challenging are the cascading "fuzz balls of light," as Neilson calls them, which have far more uncertainty with regard to direction, making it difficult to determine a neutrino's origin.

IceCube scientists applied cutting-edge machine learning techniques to ten years of observational data to identify and reconstruct those cascade events. Specifically, they devised an algorithm to compare the relative position, size, and energy of the more than 60,000 neutrino cascades recorded over that 10-year period. The resulting image revealed a series of bright spots corresponding to places in the Milky Way suspected of being high-energy neutrino sources.

"The observed excess of neutrinos from the galactic plane provides strong evidence that the Milky Way is a source of high-energy neutrinos," the authors concluded—a finding that is consistent with the distribution and expected interactions of cosmic rays with interstellar gas in the Milky Way, as revealed through gamma-ray observations.

Going forward, scientists will be looking to the next generation of neutrino detectors to advance the nascent field of neutrino astronomy further—like KM3NeT (Cubic Kilometer Neutrino Telescope) in the Mediterranean Sea, Russia's Gigaton Volume Detector (GVD, the Tropical Deep-Sea Neutrino Telescope (TRIDENT) in offshore China, Canada's P-One, and the planned IceCube-Gen2, which should be in place deep in the Antarctic ice within the next four years or so. "As next-generation observatories start unveiling the individual sources of cosmic rays, we will eventually answer the questions of their origins and what propelled them, and possibly open some new windows on the Milky Way," Luigi Antonio Fusco, a physicist at the Università degli Studi di Salerno in Italy, wrote in an accompanying perspective.

DOI: Science, 2023. 10.1126/science.adc9818  (About DOIs).